Bellows used in semiconductor-manufacturing apparatus are exposed to corrosive gases and active gases such as plasma, ozone and oxygen radicals. Stainless steels such as SUS 316 L and SUS 304 L that are generally employed as base materials for bellows do not have resistance to corrosive gases and active gases. They are therefore usually subjected to surface-treatments to achieve resistance to corrosive gases and active gases. The surface treatments include Cr2O3-passivation treatment providing excellent resistance to corrosive gases such as HCl, and fluoride-passivation treatment giving high corrosion resistance and plasma resistance.
However, Cr2O3-passivated bellows or fluoride-passivated bellows are not sufficiently resistant to corrosive gases and active gases. The use of such bellows in semiconductor manufacturing apparatus causes metallic contamination of semiconductor products such as semiconductor wafers.
For example, Cr2O3-passivated bellows show high corrosion resistance but are poor in plasma resistance. Furthermore, Cr2O3-passivated bellows cause chromium contamination when exposed to ozone or oxygen radicals, because trivalent chromium (Cr2O3) contained in the Cr2O3 passivation films is converted into volatile hexavalent chromium (CrO3).
Fluoride-passivated bellows have excellent corrosion resistance and plasma resistance. However, fluorine has a catalytic action for special material gases such as SiH4 and PH3 used in semiconductor manufacturing, and these material gases are decomposed at relatively low temperatures.
Patent Document 1 discloses Al2O3-passivated bellows that are obtained by oxidizing untreated bellows made of stainless steels of various chemical compositions, at 900 to 1200° C. in a hydrogen or inert gas atmosphere containing 1 to 10 ppm of water. The bellows are described to show high durability even when used in a highly reactive atmosphere and to be manufactured at low costs.
According to Patent Document 1, however, the chemical compositions of the stainless steels that are base materials for the bellows are broad and are not sufficiently specified, and further, the oxidation entails high temperatures of 900 to 1200° C. and consequently makes the bellows production costs high.    Patent Document 1: JP-A-2001-200346
The present invention has been made to solve the problems in the art.
It is therefore an object of the invention to provide an inexpensive method for producing bellows which show high durability even when used in a quite reactive atmosphere and which have a small catalytic function of facilitating the decomposition of special material gases such as SiH4 and PH3 used in semiconductor manufacturing.